Managing Stray Light Absorption in Integrated Photonics Devices

ABSTRACT

Fabricating a photonic integrated circuit includes fabricating structures in one or more silicon layers. At least a first silicon layer comprises: one or more photonic structures, where the photonic structures include one or more waveguides and one or more photodetectors, and one or more light absorbing structures, where at least some of the light absorbing structures include doped silicon. Fabricating the photonic integrated circuit also includes fabricating at least one waveguide in the photonic integrated circuit for receiving light into at least one of the silicon layers.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of U.S. ProvisionalApplication patent Ser. No. 62/942,358, filed Dec. 2, 2019, the entiredisclosure of which is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to managing stray light absorption in integratedphotonics devices.

BACKGROUND

Photonic integrated circuits (PICs) often include optical waveguides fortransporting optical waves around a device and into and out of variousphotonic structures (e.g., splitters, modulators, interferometers,resonators, multimode interference (MMI) couplers, photodetectors,etc.). An optical waveguide is a photonic structure that confines andguides the propagation of an electromagnetic wave. Some electromagneticwaves have a spectrum that has a peak wavelength that falls in aparticular range of optical wavelengths (e.g., between about 100 nm toabout 1 mm, or some subrange thereof), also referred to as “opticalwaves,” “light waves,” or simply “light.” These optical waveguides maybe implemented, for example, by forming a core structure from a materialhaving a higher refractive index (e.g., silicon, or silicon nitride)surrounded by a cladding (also called a “buffer”) comprising one or morematerials (or air) that have a lower refractive index. For example, thecore structure may be formed by the silicon layer over a buried oxide(BOX) layer (e.g., silicon dioxide) of a substrate, such as asilicon-on-insulator (SOI) wafer, while the cladding would be formed bythe oxide of the BOX layer and the silicon dioxide deposited on top ofthe core structure. The cladding may in some cases be formed by a singlelower-index material (or air), or by multiple different lower-indexmaterials (or air). Air can act as cladding, for example, if a corematerial is deposited on top of a cladding material without anothermaterial being deposited on top of the core material, or if a corematerial is suspended above a substrate.

SUMMARY

In one aspect, in general, an article of manufacture comprises: aphotonic integrated circuit that includes one or more silicon layers,where at least a first silicon layer comprises: one or more photonicstructures, where the photonic structures include one or more waveguidesand one or more photodetectors, and one or more light absorbingstructures, where at least some of the light absorbing structuresinclude doped silicon; and at least a first waveguide in the photonicintegrated circuit for receiving light into at least one of the siliconlayers.

In another aspect, in general, a method for fabricating a photonicintegrated circuit comprises: fabricating structures in one or moresilicon layers, where at least a first silicon layer comprises: one ormore photonic structures, where the photonic structures include one ormore waveguides and one or more photodetectors, and one or more lightabsorbing structures, where at least some of the light absorbingstructures include doped silicon; and fabricating at least one waveguidein the photonic integrated circuit for receiving light into at least oneof the silicon layers.

Aspects can include one or more of the following features.

The article of manufacture further comprises: at least a first inputport in the photonic integrated circuit for receiving first light intothe first waveguide, the first light characterized by a first intensity;and at least a second input port in the photonic integrated circuit forreceiving second light into a waveguide in one of the silicon layers,the second light characterized by a second intensity lower than thefirst intensity; wherein at least a first photodetector of the one ormore photodetectors is positioned at a location in the photonicintegrated circuit that: (1) receives a portion of the second light froma first waveguide coupled to the first photodetector, and (2) receives aportion of the first light scattered into the first photodetector from aportion of the photonic integrated circuit other than the firstwaveguide.

The first silicon layer comprises a layer of silicon in asilicon-on-insulator structure that includes a layer of silicon dioxideadjacent to the layer of silicon.

The doped silicon of one or more of the light absorbing structures ischaracterized by a dopant concentration of greater than 1018 atoms percubic centimeter.

At least some of the light absorbing structures include a structureconsisting essentially of a doped silicon structure at least partiallycovered with germanium.

The light absorbing structures include a first set of light absorbingstructures each consisting essentially of a doped silicon structure anda second set of light absorbing structures each consisting essentiallyof a doped silicon structure at least partially covered with germanium,and the quantity of light absorbing structures in the first set isgreater than the quantity of light absorbing structures in the secondset.

At least one of the photodetectors in the first silicon layer is closerto a plurality of the light absorbing structures in the in the first setthan to any of the light absorbing structures in the second set.

At least one photodetector comprises a photodiode formed at least inpart from germanium covering a portion of the doped silicon of the firstsilicon layer.

The light absorbing structures include a plurality of doped siliconstructures that each: has a cross-sectional shape that is approximatelya polygon, in a cross-sectional plane within the first silicon layer, isin proximity to neighboring doped silicon structures that together forma tiled pattern in the cross-sectional plane, and/or is separated fromneighboring doped silicon structures in the cross-sectional plane bysilicon dioxide.

Aspects can have one or more of the following advantages.

The techniques described herein provide a mechanism for absorbing straylight in a PIC using material layers and fabrication steps that arecompatible with those typically available in existing facilities thatare used for semiconductor fabrication (e.g., complementarymetal-oxide-semiconductor (CMOS) fabrication facilities). By covering asignificant portion of the area of a chip with light absorbingstructures (e.g., tiled structures or uniform/fully dense structures),the amount of stray light left to spread to functional photonicstructures on the chip can be effectively reduced, with minimal or noimpact on cost. In some fabrication facilities, there may be designrules imposed on the fabrication process that state that open/unusedareas of a mask need to be filled with tiled structures in order tomaintain a certain percentage fill of the mask area. In some cases, such“tiling” (also called “filler patterns”) may be used for mechanicalpurposes, such as reducing stress, increasing uniformity of structuredensity, or improving uniformity of etching results. Taking advantage ofthese structures for an additional purpose of absorbing stray light canimprove optical performance of the devices on the chip with minimalnegative impact on yield. Some examples of the devices on a PIC chipthat could benefit from such a reduction in stray light include opticalcoherent receivers, coherent transmitters, and tap photodetectors, allof which benefit in some way from the extended dynamic range that wouldbe achievable by reducing stray light. For example, in someimplementations, the dynamic range could be increased up to 5-7 dB ormore depending on tiling density and device configuration. There arealso potential cost benefits that could be achieved by reducing the needfor other, potentially more expensive, techniques for reducing ormanaging the effects of stray light, as described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Itis emphasized that, according to common practice, the various featuresof the drawings are not to-scale. On the contrary, the dimensions of thevarious features are arbitrarily expanded or reduced for clarity.

FIG. 1 is a schematic diagram of a portion of photonic integratedcircuit illustrating the spread of stray light associated with anoptical wave being coupled into an input port.

FIG. 2 is a schematic diagram showing a portion of a photonic circuitshowing an arrangement of light absorbing structures in the generalvicinity of a waveguide and a photodetector.

DETAILED DESCRIPTION

One of the reasons that it may be convenient to fabricate photonicintegrated circuits using fabrication techniques used for electrical(e.g., CMOS) integrated circuits (ICs) is that optical signals carriedby optical waves can be used in the same chip where electrical signalsare carried by an electrical current or voltage. In some cases, avariety of photonic and electronic devices can be integrated into thesame chip. One kind of device that is used both in chips that containmostly PIC devices and chips that integrate PIC and IC devices is aphotodetector (or a photoreceiver) that converts an optical signal intoan electrical signal. An example of a platform that can be used for PICand IC devices is a silicon photonics (SiP) platform. A SiP chip can befabricated using a silicon wafer (e.g., a silicon-on-insulator (SOI)wafer) that is processed using various standard fabrication processsteps and diced to yield individual SiP dice.

A photodetector can be formed from a photonic structure called aphotodiode, which is able to convert the photons of a received opticalwave into a photocurrent. This photocurrent can then be furtherprocessed by an electronic circuit to provide an output current or anoutput voltage (e.g., in a transimpedance amplifier). Such photodetectordevices formed using photodiodes can be used, for example, to detect avariety of types of optical signals coming from various types ofphotonic devices (e.g., an optical modulator), which may be formed fromspecific photonic structures (e.g., a Mach-Zehnder interferometer, or aring resonator). In order to monitor the activity of these photonicdevices, a small amount of light can be coupled out of a waveguide usinga directional coupler or a multi-mode interference splitter, forexample, and sent to these monitoring photodetectors.

In some cases, it may be beneficial for a device receiving an opticalsignal to have a large dynamic range, such that both strong signals(carried on an optical wave with a relatively high optical power) andweak signals (carried on an optical wave with a relatively low opticalpower) are able to be detected. However, if the optical wave beingdetected has a low optical power, then it may be more vulnerable tostray light leaking into the detector, or into a waveguide coupled tothe detector, from somewhere other than an intended source. Onepotential source of stray light may occur due to a mismatch betweenintensity profiles of different guided modes of different respectivewaveguides that are coupled to each other. For example, if there isimperfect alignment between an optical fiber and an input port of a PIC,some of the power in the mode of the optical fiber that is not perfectlymatched to the mode of the input port can make its way into a substrateof the PIC, or the silicon dioxide cladding layers that surround theoptical waveguides. Similarly, if different types of waveguides on a PICare coupled without a sufficiently adiabatic transition between thedifferent waveguides, there may be some stray light that is lost to asurrounding substrate. For example, if the PIC comprises a SiP die,light may leak into the silicon substrate, the BOX layer of the SOIsubstrate, or the silicon dioxide that has been deposited above a layerof photonic structures. Even if light is received from an output mode ofa laser or other light source integrated in the SiP (i.e., withoutrequiring an explicit input port requiring alignment), stray light fromthe source outside of the output mode may also leak out of the sourceand into the SiP die. Some structures in the SiP die, such asimperfectly terminated unused ports of couplers, may also scatter straylight. Another potential source of stray light is scattering fromwaveguide surfaces that are not sufficiently smooth (e.g., from roughsidewalls of a ridge waveguide or a rib waveguide). While light can alsobe scattered due to route changes in a waveguide (e.g., bends that havea radius that too large to maintain total internal reflection of lightin a guided mode), the bends in a waveguide are typically designed to belarge enough to avoid such losses. Such stray light from any of thesecauses can then undesirably propagate to and illuminate some or all ofthe photodiodes in the PIC.

FIG. 1 shows a portion of PIC in a SiP die 100 that includes structuresthat would be present in a typical integrated system, such as an opticalfront end of a coherent receiver, and illustrates an example of theeffects of stray light. In this example, some optical structures andother fabricated structures (e.g., metal conducting paths) are not shownfor clarity. But, this example does show an input port 102 for couplingthe local oscillator (LO) light into an optical waveguide 110, andanother input port 104 for coupling the receiver signal (SIG) light intoanother optical waveguide 112 (with only portions of the opticalwaveguides 110 and 112 being shown). The propagation of stray light overportions of the SiP die 100 are represented by different stray lightregions 105A, 105B, and 105C in which stray light would have differentrespective ranges of intensity, with the region 105A having the highestintensity of stray light, and the regions 105B and 105C having lowerintensities of stray light. While the actual amount of loss representedby the light that spreads into the stray light regions 105A, 105B, and105C may be relatively small (e.g., 0.1%), even a small loss from asufficiently intense optical wave can result in a significant amount ofstray light. In this example, the wavelength of both the LO light andthe SIG light is in the infrared (IR) portion of the electromagneticspectrum, around 1550 nm.

The stray light regions 105A, 105B, and 105C in this example are mostlya result of to the relatively intense unguided LO light that is leakinginto surrounding portions of the substrate and upper-cladding, duemainly to misalignment at the input port 102. The brightest stray lightregion 105A would be near the initial path of the LO light, but theother stray light regions 105B and 105C could still receive significantamounts of the leakage LO light, which can cause problems for signalmeasurement of the SIG light. While the placement of these or otherdetectors would ideally be in relatively dark regions that are not asintensely illuminated by stray light as the stray light regions 105A-C,such selective placement may not always be possible. So, it is useful toabsorb as much of this stray light as possible within as many of thestray light regions as possible in order to make the whole SiP die 100darker.

In this example, there are tap photodiodes (PDs) 106 that are used todetect the amount of SIG light received at the input port 104 that isdelivered by an optical waveguide before and after sets of in-linevariable optical attenuators (VOAs) 108. Such measurements can be usedfor a variety of applications (e.g., input or output referred opticalpower monitoring, Mach-Zehnder Modulator (MZM) quadrature lockingcircuits, etc.). These tap PDs 106 are in the stray light region 105C ofthe SiP die 100 that is being illuminated by a significant amount of theleaked LO light. This illumination by stray light may be undesirablesince it can limit the dynamic range attainable by the tap PDs 106. Forexample, if the SIG light is relatively weak after attenuation and theLO is relatively strong, the leaked LO light that reaches a tap PD maybe strong enough generate noise that degrades the SIG light measurement.

In order to reduce the amount of stray light propagating in one or moreof the different layers of a PIC, structures can be incorporated intothe PIC to absorb at least a portion of the stray light. Such lightabsorbing structures are able to reduce the intensity of any remainingstray light that may reach the photodiodes. In some implementations,techniques and materials that are compatible for use in CMOS fabricationfacilities can be configured for forming light absorbing structures in aPIC device such as a SiP die (or in an integrated PIC/IC device). Forexample, some of the materials that can be used to generate lightabsorbing structures in the layers of a PIC fabricated using a CMOSprocess are: doped silicon, germanium, and metals, as described in moredetail below.

Structures formed from doped silicon can be effective at absorbing straylight. Doping a material such as silicon can be accomplished as part ofa standard CMOS fabrication process by introducing atoms of a foreignmaterial (also called “impurities”), which can be of two differenttypes: an n-type dopant (which provides free electrons as negativecharge carriers), or a p-type dopant (which provides mobile holes aspositive charge carriers). Examples of p-type dopants include boron,gallium, or aluminum. Examples of n-type dopants include arsenic,phosphorous, or antimony. A silicon layer that is to be doped (as partof the CMOS fabrication process) can be, for example, an initial siliconlayer that is also used to form waveguides and other photonicstructures, or another silicon layer that is grown during the CMOSfabrication process. Whatever silicon layer(s) are used for doping, itis useful if the silicon is as highly doped as possible to render it asabsorbing as practically possible. The concentration of a dopant can becharacterized by different degrees of concentration, which can beassociated with corresponding symbols (P for p-type, and N for n-type)within various quantitative ranges. A “P” or “N” designation ofconcentration is a moderate degree of doping (e.g., a concentration ofless than 10¹⁸ atoms per cubic centimeter). A “P⁺” or “N⁺” designationof concentration is a heavy degree of doping (e.g., a concentration ofbetween about 10¹⁸ to 10²⁰ atoms per cubic centimeter). A “P⁺⁺” or “N⁺⁺”designation of concentration is an even heavier degree of doping (e.g.,a concentration of greater than about 10²⁰ atoms per cubic centimeter).A P⁺⁺ or N⁺⁺ concentration is effective for the absorbing structures ina silicon layer, whereas undoped silicon has relatively littleabsorption (which is why it is typically used to form opticalwaveguides).

Germanium can be grown onto portions of a wafer using an epitaxyprocess. The germanium can be grown in thicknesses from 100 nm to a fewmicrons. In some implementations, a germanium structure is grown on topof a silicon structure. For example, the silicon structure can be in theform of a tile (e.g., a square tile) that has been etched within asilicon layer, and the germanium can be in the form of a smaller tilepositioned on top of the silicon tile. Germanium is an effectiveabsorbing material, and thus may be able to absorb a significant portionof the stray light. The silicon underneath the germanium can be doped toany degree, or can remain undoped. There may be rules associated withthe CMOS fabrication process that limit the quantity of germanium thatcan be grown (e.g., due to the stress caused by the lattice mismatch)and/or the proximity of the germanium to other photonic structures,including photodiodes, also formed using germanium. The germanium can begrown on silicon with various amounts of doping, from undoped to veryhighly doped.

Metal can also be used as an absorber. An electromagnetic wave having awavelength in the visible or infra-red part of the spectrum propagatesinto metals with a very high attenuation. This is characterized by thehigh imaginary part of the refractive index of metals. Metal depositedon top of silicon can thus act as an absorber. Although a largerfraction of an incident optical wave is reflected according to theFresnel equation, a small fraction is transmitted and absorbed insidethe metal.

The silicon layer used for the waveguide layer (e.g., the silicon layerabove the BOX layer of a SOI wafer) is typically about 220 nm to about500 nm thick. Doping this relatively small thickness of a silicon layermay not absorb stray light as effectively as germanium, however, thestray light will pass through the light absorbing structures in thesilicon layer multiple times, so the amount of absorption can still beeffective for mitigating the effects of stray light on photonic devicecharacteristics such as dynamic range. Alternatively, a potentiallythicker silicon layer, deposited by subsequent CMOS process steps, canbe used to form the doped silicon light absorbing structures. Thethicker the silicon, and the more it is doped, the higher the absorptionwill be.

Referring to FIG. 2, a portion of a photonic circuit shows anarrangement of light absorbing structures in the general vicinity of awaveguide 204 and a photodetector 206. Some CMOS fabrication rules mayrequire certain tiling patterns be used for various purposes. The rulesmay specify density of tiles, dimensions and shapes of tiles, or variousother characteristics, for example, for certain other mechanical orfabrication process purposes for which the tiling is used. The tilingpatterns may fill otherwise unused area in a PIC, such as area aroundthe waveguide 204, as shown in FIG. 2. In this example, the tiles have asquare shape and even spacing, but in other examples any polygons thatare able to be arranged into a tiled pattern may be used, and the tilingpattern may have different spacing between adjacent tiles in differentdimensions. The trenches 208 between the tiles may be filled with amaterial (e.g., silicon dioxide) used for a layer deposited or grown ontop of a silicon layer from which the tiles are formed.

However the tiles are arranged geometrically, such tiling structures canbe used to fabricate light absorbing structures, for example, by dopingsilicon tiles and/or by adding germanium to the tiles. In this example,some light absorbing structures are formed from a doped silicon tile 200without any germanium in contact with the doped silicon, and some lightabsorbing structures are formed from a silicon tile 200 with a germaniumtile 202 grown on a portion of the surface of the silicon tile 200,where the doping of that tile could vary from undoped to very highlydoped. In this example, the light absorbing structures closest to thephotodetector 206 use just the doped silicon tile 200 without germaniumdue to a fabrication design rule that limits how close germanium tilingcan be to photonic structures that contain germanium, such as thephotodetector 206. But, with sufficient spacing from the photodetector206 (e.g., a distance of at least 100 microns), the light absorbingstructures can use both the doped silicon tile 200 and the germaniumtile 202 (or the germanium tile 202 without the doped silicon tile 200).

In some cases, changes to the tiling pattern can be made while stayingwithin the fabrication rules (or by modest changes to the fabricationrules) to optimize size and/or placement of the light absorbingstructures. For example, larger tiles can be used to cover a largerpercentage of a large otherwise unused area, or smaller tiles can beused to more closely conform to the edges of an area that has a higherdensity of device photonic structures.

The following experimental results provide quantitative estimates of theeffects of the light absorbing structures on the performance of certainphotonic devices, by way of example only. A variety of performanceresults may be obtained for different levels of doping, differentarrangement of tiles or other shapes and/or arrangements of lightabsorbing structures. These estimates are based on results fromprototype SiP dies on which the optical front end of a coherent receiverhas been fabricated.

A first prototype SiP die included germanium tiles without dopedsilicon, to assess the effects of the germanium. With 8% tiling density(averaged over the entire surface of the SiP die), the improvement indynamic range was about 1.5 dB. In some experiments, the improvement indynamic range was approximately directly proportional to the tilingdensity. The tiling density can be increased to significantly higherthan 8% in some cases, depending on the density of photonic structures.For example, in a relatively standard device configuration, thegermanium density can be increased to about 25% to 30%, which wouldprovide about a 5 dB to 7 dB increase in dynamic range. A secondprototype SiP die included P⁺+ doped silicon tiles without germanium.With 20% tiling density, the improvement in dynamic range was about 1.0dB. In a relatively standard device configuration, the doped silicontiling density can be increased to about 30% to 40%, which would providea 1.5 dB to 2.0 dB increase in dynamic range. The doped silicon tilingalone without germanium is not as effective as structures that includegermanium in absorbing (and thus reducing) the stray light, but thedoped silicon tiling could be used in SiP/CMOS process flow in whichgermanium is not available. Alternatively, doped silicon tiling could beused in combination with germanium tiling, as described above, and it isexpected that the benefits of each type of tiling add to increase thetotal absorption and thus and improve the overall dynamic range.

The absorbing of stray light using light absorbing structures asdescribed herein can be combined with various other techniques forabsorbing stray light and/or mitigating the effects of remaining straylight. For example, in some implementations, the top surface of a SiPdie can be covered with a light absorbing adhesive (e.g., epoxy) thatwill effectively absorb additional stray light. However, dispensingadhesives on small PIC chips can be relatively complex due to the needto cover the maximal surface, while avoiding any wirebonds, opticalbonding surfaces, and RF lines, as any adhesive on these surfaces canaffect reliability, performance, and processing. The adhesive dispensingprocess may also be difficult to control, and thus could have a notableimpact on yield. The additional step of applying the adhesive may alsoadd cost to the product with extra dispensing equipment and curingoperations. For implementations that do not apply such an adhesive,standard steps in a CMOS fabrication process can be used, as describedherein, for potentially an insignificant cost or yield impact.

If there is a predictable amount of stray light that will remain, evenif some stray light is absorbed, there are some techniques that mitigatethe effects of the stray light using calibration tables. For example,with a single photodetector, the effect of stray light and reverse-biasleakage (also called “dark current”) can be reduced to an extent usingcalibration tables to measure the responsivity of the stray light anddark current as a function of wavelength and temperature. Theseextensive calibration tables come at the real cost of notably increasedtest time on every manufactured part. So, for implementations that donot use such calibration tables, there may also be significant costreduction. There may also be difficulty in accurately predicting thequality of the calibration tables as the products age because of thecomplex nature of the optical interference signal.

While the disclosure has been described in connection with certainembodiments, it is to be understood that the disclosure is not to belimited to the disclosed embodiments but, on the contrary, is intendedto cover various modifications and equivalent arrangements includedwithin the scope of the appended claims, which scope is to be accordedthe broadest interpretation so as to encompass all such modificationsand equivalent structures as is permitted under the law.

What is claimed is:
 1. An article of manufacture, comprising: a photonicintegrated circuit that includes one or more silicon layers, where atleast a first silicon layer comprises: one or more photonic structures,where the photonic structures include one or more waveguides and one ormore photodetectors, and one or more light absorbing structures, whereat least some of the light absorbing structures include doped silicon;and at least a first waveguide in the photonic integrated circuit forreceiving light into at least one of the silicon layers.
 2. The articleof manufacture of claim 1, further comprising: at least a first inputport in the photonic integrated circuit for receiving first light intothe first waveguide, the first light characterized by a first intensity;and at least a second input port in the photonic integrated circuit forreceiving second light into a waveguide in one of the silicon layers,the second light characterized by a second intensity lower than thefirst intensity; wherein at least a first photodetector of the one ormore photodetectors is positioned at a location in the photonicintegrated circuit that: (1) receives a portion of the second light froma first waveguide coupled to the first photodetector, and (2) receives aportion of the first light scattered into the first photodetector from aportion of the photonic integrated circuit other than the firstwaveguide.
 3. The article of manufacture of claim 1, where the firstsilicon layer comprises a layer of silicon in a silicon-on-insulatorstructure that includes a layer of silicon dioxide adjacent to the layerof silicon.
 4. The article of manufacture of claim 1, where the dopedsilicon of one or more of the light absorbing structures ischaracterized by a dopant concentration of greater than 10¹⁸ atoms percubic centimeter.
 5. The article of manufacture of claim 1, where atleast some of the light absorbing structures include a structureconsisting essentially of a doped silicon structure at least partiallycovered with germanium.
 6. The article of manufacture of claim 5, wherethe light absorbing structures include a first set of light absorbingstructures each consisting essentially of a doped silicon structure anda second set of light absorbing structures each consisting essentiallyof a doped silicon structure at least partially covered with germanium,and the quantity of light absorbing structures in the first set isgreater than the quantity of light absorbing structures in the secondset.
 7. The article of manufacture of claim 6, where at least one of thephotodetectors in the first silicon layer is closer to a plurality ofthe light absorbing structures in the in the first set than to any ofthe light absorbing structures in the second set.
 8. The article ofmanufacture of claim 1, where at least one photodetector comprises aphotodiode formed at least in part from germanium covering a portion ofthe doped silicon of the first silicon layer.
 9. The article ofmanufacture of claim 1, where the light absorbing structures include aplurality of doped silicon structures that each has a cross-sectionalshape that is approximately a polygon, in a cross-sectional plane withinthe first silicon layer.
 10. The article of manufacture of claim 9,where the plurality of doped silicon structures are each in proximity toneighboring doped silicon structures that together form a tiled patternin the cross-sectional plane.
 11. The article of manufacture of claim10, where the plurality of doped silicon structures are each separatedfrom neighboring doped silicon structures in the cross-sectional planeby silicon dioxide.
 12. A method for fabricating a photonic integratedcircuit, the method comprising: fabricating structures in one or moresilicon layers, where at least a first silicon layer comprises: one ormore photonic structures, where the photonic structures include one ormore waveguides and one or more photodetectors, and one or more lightabsorbing structures, where at least some of the light absorbingstructures include doped silicon; and fabricating at least one waveguidein the photonic integrated circuit for receiving light into at least oneof the silicon layers.
 13. The method of claim 12, further comprisingfabricating in the photonic integrated circuit: at least a first inputport for receiving first light into the first waveguide, the first lightcharacterized by a first intensity; and at least a second input port forreceiving second light into a waveguide in one of the silicon layers,the second light characterized by a second intensity lower than thefirst intensity; wherein at least a first photodetector of the one ormore photodetectors is positioned at a location in the photonicintegrated circuit that: (1) receives a portion of the second light froma first waveguide coupled to the first photodetector, and (2) receives aportion of the first light scattered into the first photodetector from aportion of the photonic integrated circuit other than the firstwaveguide.
 14. The method of claim 12, where the first silicon layercomprises a layer of silicon in a silicon-on-insulator structure thatincludes a layer of silicon dioxide adjacent to the layer of silicon.15. The method of claim 12, where the doped silicon of one or more ofthe light absorbing structures is characterized by a dopantconcentration of greater than 10¹⁸ atoms per cubic centimeter.
 16. Themethod of claim 12, where at least some of the light absorbingstructures include a structure consisting essentially of a doped siliconstructure at least partially covered with germanium.
 17. The method ofclaim 16, where the light absorbing structures include a first set oflight absorbing structures each consisting essentially of a dopedsilicon structure and a second set of light absorbing structures eachconsisting essentially of a doped silicon structure at least partiallycovered with germanium, and the quantity of light absorbing structuresin the first set is greater than the quantity of light absorbingstructures in the second set.
 18. The method of claim 17, where at leastone of the photodetectors in the first silicon layer is closer to aplurality of the light absorbing structures in the in the first set thanto any of the light absorbing structures in the second set.
 19. Themethod of claim 12, where at least one photodetector comprises aphotodiode formed at least in part from germanium covering a portion ofthe doped silicon of the first silicon layer.
 20. The method of claim12, where the light absorbing structures include a plurality of dopedsilicon structures that each: has a cross-sectional shape that isapproximately a polygon, in a cross-sectional plane within the firstsilicon layer, is in proximity to neighboring doped silicon structuresthat together form a tiled pattern in the cross-sectional plane, and isseparated from neighboring doped silicon structures in thecross-sectional plane by silicon dioxide.